Polymeric foams can be generally classified as either closed-cell foams or as open-cell foams. Open-cell foams can be used as a matrix to contain various liquids and gases. They are capable of various industrial applications such as, for example, use in wipes and diapers, as carriers and ion exchange resins For some of these applications, it is desirable to have porous crosslinked polymer blocks which have a very low density and a high capacity of absorbing and retaining liquids. Such high absorption capacity, low density, porous polymer blocks can be prepared by polymerizing a specific type of water-in-oil emulsion known as high internal phase emulsion (HIPE) having relatively small amounts of a continuous oil phase and relatively greater amounts of an internal water phase. Further, other properties such as good wicking and good retention of liquid under load (i.e., low compressive strain or resistance to compression deflection) are also desirable for use as an absorbent.
Typically, these high absorption capacity, low density foams are prepared by forming a high internal phase water-in-oil emulsion in the presence of a surfactant and polymerizing the monomers in the oil phase of the emulsion with a polymerization initiator at a temperature around 60.degree. C. for about 8 hours. However, it has been found that in order to obtain foams with better properties, curing must be conducted for 16 hours or longer at a temperature of 60.degree. C. Further, to produce these foams in a continuous process, it is desirable to heat the emulsion rapidly and to reduce the curing time Therefore, it will be advantageous to reduce the curing time and to heat the emulsion rapidly without significantly affecting the resulting foam properties.
However, it has been found that by raising the temperature rapidly above about 65.degree. C., the emulsion deteriorates thereby affecting the resulting foam products Therefore, it will be desirable to be able to rapidly heat the emulsions and/or reduce the curing time without degrading the water-in-oil emulsion and adversely affecting the foam product properties.
It is therefore an object of the present invention to provide a process to prepare low density, porous crosslinked polymeric foams with improved absorption properties. It is another object of the present invention to provide a process to reduce curing time and/or to allow rapid heating without substantially degrading the water-in-oil emulsion.